just the single idea of having a Part IV made already makes me angry, and all these ideas make it all worse. not that they are bad ideas, but I just don't see none of the working.
this one about an intercut between Vincent and Sonny is just the exact same thing from Part II, and since one of the greatest things about the Godfather films is how original they are, it would be a shit, in my humble opinion.
and making a Part IV withouth Al Pacino sounds like an absurd to me.